Coaxial gravity meter

ABSTRACT

A gravity meter having two masses which are both fixed to independently rotate about the same axis. Strain gauges are connected from one mass to its center of rotation to detect centrifugal and gravitational forces acting on the mass and the angular velocities of both masses are adjusted to provide an AC null in the output of the strain gauges. When such a null is attained, the centrifugal force developed by the masses balances the force due to gravity. Since the angular velocities of the masses are directly related to the centrifugal force, a measurement of the angular velocity provides a measure of gravity.

United States Patent 72] Inventors Henry P. Kalmus; PrimaryExaminerRichard C. Queisser Billy M. Horton, both of Washington, D.C.Assistant Examiner-Herbert Goldstein [21 Appl. No. 854,472Attorneys-l-larry M. Saragovitz, Edward J. Kelly, Herbert [22] FiledSept. 2, 1969 Berl and J. D. Edgerton [45] Patented Aug. 17, 1971 [73]Assignee The United States of America as represented by the Secretary ofthe Army ABSTRACT: A gravity meter having two masses which are [54]COAXHAL GRAVITY'METER both fixed to independently rotate about the sameaxis. Strain 8 Claims 4 Drawing Figs gauges are connected from one massto 1ts center of rotation to detect centrifugal and gravltanonal forcesacting on the [52] U.S. Cl 73/382 mass and the angular velocities f bmasses are adjusted to f Cl Golv 7/00 provide an AC null in the outputof the strain gauges. When Flew of Search 73/382 such a null isattained, the centrifugal force developed by the masses balances theforce due to gravity. Since the angular [56] References cued velocitiesof the masses are directly related to the centrifugal UNITED STATESPATENTS force, a measurement of the angular velocity provides a mea-3,336,806 8/1967 Kalmus 73/382 sure ofgravity.

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A TTORNEYS COAXIAL GRAVITY METER BACKGROUND OF THE INVENTION Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment to us of any royalty thereon.

This invention relates to gravity meters, and more particularly to adynamic gravity meter which utilizes centrifugal force of a magnitudethat may be accurately determined with great precision to balance thegravitational forces to be measured.

A precursor of the present invention is described in U.S. Pat. No.3,336,806, issued Aug. 22, 1967, to Kalmus and the disclosure of thatpatent is incorporated herein by reference.

In that patent, the state of the art of gravity measurement devices issummarized and three types of gravity meters, i.e., dynamic gravitymeters, stable gravity meters and astatized gravity meters, aredescribed. These prior art gravity meters are subject to a numberofdisadvantages, e.g., the tendency to be subject to zero drift, thenecessity to be calibrated frequently and the propensity to be affectedby variations in temperature and pressure. As a result, great care mustbe exercised in selecting materials with low-temperature coefficientsand continual barometric readings must be made to compensate for theeffect of the density of the air. Alternatively, the mass systems ofthese prior art gravity meters could be placed in a sealed or evacuatedchamber. Additionally, in almost all cases these gravity meters areextremely sensitive and subject to misalignment precluding their use inan adverse environment such as in aircraft or ships.

The gravity meter described in the above-noted Kalmus patent obviatedthese aforementioned disadvantages of prior art gravity meters byproviding a dynamic gravity meter which utilizes two rotating masses. Afirst mass is rotated about a fixed point and a second mass is rotatedabout the first mass. A strain gauge in the radius arm of the first massand connected between the fixed point and the mass detects the internalforces resulting from the rotation of the masses and the angularvelocities of both masses are regulated to provide an AC null in theoutput of the strain guage.

In that Kalmus gravity meter, two radius arms are employed, one toconnect the first mass to the fixed point and the other to connect thesecond mass to the first mass. Of necessity this gravity meter requiressufficient space to permit the rotation of both masses. Additionally,that device, in order to reduce the main bearing load, requires that thesystem be balanced by providing an equal mass system extending from thefixed point on an extension of the radius arm for the first mass. Theextension of the mass system thus increases the space requirements forthe gravity meter.

It is therefore an object of the present invention to provide animproved dynamic gravity meter which having once been calibrated permitsthe absolute measurement of gravity repeatedly without recalibration.

It is another object of the present invention to provide an improveddynamic gravity meter in which a force due to gravity is continuallycompared with an accurately mcasureablc centrifugal force by an AC nullmethod.

It is a further object of the present invention to provide an improvedgravity meter which is relatively insensitive to variations intemperature and pressure.

SUMMARY OF THE INVENTION According to the present invention, theforegoing and other objects and advantages are attained by providing agravity meter having two masses which are both rotated independentlyabout the same axis. Strain gauges are connected between one of themasses and the central axis of rotation to measure the resultantinternal forces acting on the rotating mass. The angular velocities ofboth masses are adjusted to provide an AC null in the output of thestrain gauges, and

when such a null is attained, the centrifugal force developed by therotating masses balances the force due to gravity. Since the angularvelocity is directly related to the centrifugal force, a measurement ofthe angular velocity provides a measure of gravity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view, partlyin section, showing a diagrammatic representation of the presentinvention;

FIG. 2 is a vertical sectional view taken on line 2-2 of FIG.

FIG. 3 is a representation of the mass system of the present inventionshowing the relationship between the two rotating members; and

FIG. 4 is a vertical view, partly in section, showing an alternativeembodiment for the rotor drive.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings,wherein like reference nu merals designate identical or correspondingparts throughout the several views, and more particularly in FIG. I ashaft 10 is shown rotatably mounted in bearing members 12 and 14. Avariable speed motor 16, coupled to shaft 10 provides a power source torotate shaft 10 at a controlled rate. A rotor 18 having a hub portion 20with a bore 22 therethrough and radially extending flanges 24 and 26with axially extending lip segments 28 and 30, respectively, isconcentrically disposed about shaft 10. The diameter of bore 22 isslightly greater than the diameter of shaft 10 and, hence, rotor 18 fitsfreely over shaft 10. Rotor I8 is coupled to shaft 10 by means ofopposed pairs of radially extending ribs 32 and 34, respectively, (seeFIG. 2 as well) and single opposed wire strands 36-42 which are fixedbetween the peripheral surface ofshaft I0 and the lip segments 28 and30. Strain gauge transducers 44-50 are mounted, respectively, in each ofthe four single strands 36 42 to record the centrifugal forces acting onrotor 18 when it is rotated.

The opposed pairs of ribs 32 and 34 are sufficiently rigid to supportthe rotor 18 so that motion of rotor 18 only in the direction of wires36 and 38 is permitted.

Concentrically disposed about hub 20 of rotor 18 is a second rotor 52having a bore 54 of a larger diameter than the outside diameter of hub20 so that rotor 52 is free to rotate relative to rotor 18. An extensionarm 56 extends radially outwardly from the periphery of rotor 52 andsupports an eccentric weight 58.

Rotors 18 and 52 are driven independently so that each may rotaterelative to the other. Rotor 18, since it is fixed to shaft 10, isdriven by variable speed motor 16. Rotor 52 may be driven in anyconvenient manner, i.e., reaction jets of air, synchronous electricmotors or pressure gearing, but preferably by an optical drive system.To utilize an optical drive system the periphery of rotor 52 is providedwith a sawtooth configuration comprising a pwrality of circumferentiallyspaced sawtooth-shaped protrusions 60 with ch sawtooth protrusion havinga radially directed rear surface 02 which is blackened so as to absorbradiation while all other surfaces of the protrusion are highly polishedso as to reflect radiation The rotor chamber is uniformly flooded withlight in all directions from a number of light sources 64. The reactiveforce of the gas molecules near the bla kened surfaces which absorbradiation emitted by light sources 64 make the rotor advance in adirection opposite to that of a cutting sawtooth blade. The speed andphase angle of the rotor are controlled by changing the intensity of theillumination, hence, the intensity of the radiation, emanating fromlight sources 64.

To determine the speed of rotation and phase angle of rotor 52 a lightsource 63 is positioned so that its beam of light is coaxially directedthrough a protrusion 60 and the interruptions to this beam of light canbe calibrated and correlated to the rotors speed, for example by acounter 65. The speed of rotor 18 can be accurately determined by therotation of shaft l and its speed can be controlled by controlling thespeed of motor 16.

In the alternative, the speed control of rotors l8 and 52 may beaccomplished through the use of a magnetic recording signature in thesame manner as is described in the abovementioned Kalmus patent forcontrolling the speed of rotation of the rotors in the precusor device.

In order to insure that rotor 52 rotates freely with respect to rotor18, an air bearing is provided in hub of rotor 18. Shaft 10 is providedwith an axial passage 70 which terminates in a branched T-passage 72 sothat air under pressure can be passed through shaft 10 and out theopenings 74 and 76 formed by the juncture of passage 72 and theperipheral surfaces of shaft 10. Suitable fittings 78 and 80 areprovided to secure one end of air supply lines 82 and 84 thereto. Lines82 and 84 are sufficiently flexible so that they do not provide a rigidconnection between rotor 18 and shaft 10 and do not affect the forcesimparted to the radial strands 3642. Alternately, lines 82 and 84 can bedeleted if ribs 32 and 34 are designed as flat hollow tubes which carrythe air from the shaft 10 to rotor 18. The other end oflines 82 and 84are secured by fittings 86 and 88, respectively, to rotor 18 andcommunicate with passages 90 and 92, respectively, thereby to allowfluid communication between passages 70-72 and 9092. Each passage 90 and92 includes a plurality of branched passages 94 which are oriented so asto direct the air under pressure to impinge upon the surface of bore 54of rotor 52. Hence, the air under pressure provides an air bearing andinsures relatively frictionless relative rotation between rotors l8 and52.

It has also been found that by placing the two rotors so that theyrotate about a common axis, rotor 52 carrying eccentric mass 58 can bemade with a relatively small diameter so that the stability of thelength of the radius arm from the center of rotation to the center ofthe eccentric mass 58 can be maintained. In addition, with the presentconstruction, the tension ofthe wires which serve simultaneously assupport for rotor 18 and as strain gauges can be adjusted easily.

Reference is now made to FIG. 3, wherein there is shown a diagrammaticrepresentation of the gravity meter of the present invention. The tworotors 18 and 52 are shown rotating about the common axle or shaft 10.Rotor 18 rotates with an angular velocity of w, and rotor 52 with anangular velocity of (0 Rotor 18 carries two transducers 44 and 46 andthe transducer outputs are combined in such a way that a positivevoltage is obtained if transducer 44 is under pressure and a negativevoltage is obtained if transducer 46 is under pressure and rotor 52includes an extension to which is fixed a mass 58.

Let the mass of all movable parts (rotor 18 and rotor 52) be M and themass of the extension 58 of rotor 52 be m. There are two forces actingon the transducers; F due to gravity and F due to centrifugalacceleration.

F =mw r COS( tu -(oat, where r is the radius from the center of shaft 10to the center of mass 58. For zero transducer output: F =F,. Let al be2m, and let (9 Therefore:

Since r, m and M are all constants and are fixed, an adjustment of theangular velocities w, and (0 so that the transducer outputs are zero andi is twice (u, will result in an accurate determination of g, theacceleration due to gravity.

With reference now to FIG. 4, there is shown an alternative arrangementof the drive for concentric rotors 118 and 152 about drive shaft 110.Shaft 110, as is shaft 10 in the embodiment of FIG. 1, is journaled inbearing members 112 and 114 and is rotated therein by a motor 116coupled thereto. A disk 200 is fixed about shaft 110 and carries amagnetic signature impressed thereon its surface. A transducer 202 isengaged to the disk 200 in contact with the magnetic signature andproduces a signal with a frequency which is proportional to the angularvelocity ofshaft 110.

This signal is fed through connections in shaft (not shown) and throughwires 204 between shaft 110 and rotor 118 to transducers 206, 208, 210and 212 which are positioned to extend inwardly from flanges 124 and 126of rotor 118. On the radial surface of rotor 152 is a magnetic signaturewhich is identical to the magnetic signature on disk 200. Transducers206, 208, 210 and 212 are positioned to engage the magnetic signature onrotor 152. Hence, the signal imposed on the surface of rotor 152 by thetransducers 206 212 will cause rotor 152 to rotate with respect to rotor118 with an angular velocity which is equal to the angular velocity ofrotor 118. Thus, if rotor 118 is rotating with an angular velocity of wand rotor 152 rotates with respect to rotor 118 with the same angularvelocity, then rotor 152 will rotate with an angular velocity of 2m.

It is advantageous to produce a three-phase signal in transducer 200for, in this way, the electrical connection between the sending andreceiving transducers act like an electromechanical shaft. It is alsobest to keep the synchronizing signal very small and to amplify it bymeans of a transistor amplifier (not shown) within rotor 118. It is alsoto be understood that instead of using wires 204 as signal-carryingmeans, the signal and the amplifier supply voltage can be fed to rotor 118 by inductive means, by modulated RF energy or by a modulated lightbeam With reference again to FIG. 4, it is seen that rotor 152 isprovided with a hole 220 which is bored therein along a portion of rotor152. This hole is filled with a material which may be of a greater orlesser density than the material of rotor 152 and when so filled willcause the rotor to act like the unbalanced or eccentric mass 58 in theembodiment of FIG. 1. This alternative has the advantage that windresistance to the rotation of rotor 152 is lessened.

It is thus seen that a dynamic gravity meter is provided which can beused to accurately determine the force due to gravity without regard tobarometric pressure, thermal expansion of parts of the meter and theneed for frequent calibrations.

We claim:

1. A gravity meter which utilizes centrifugal force ofa magnitude thatmay be accurately determined with great precision to balance thegravitational force to be measured comprising:

a. a first rotor having a fixed and known mass which is free to rotatein a single plane of rotation about a fixed point,

b. a second rotor having a fixed and known mass including aneccentrically disposed mass, c. said second rotor being coaxiallydisposed about and supported by said first rotor and free to rotate in asingle plane of rotation about said fixed point,

. first drive means for rotating said first rotor,

e. second drive means for rotating said second rotor and said eccentricweight,

f. control means for adjusting the angular velocities of said first andsecond rotors,

g. transducer means supported b strands located between said first drivemeans and said first rotor for c nsing the resultant component ofgravitational and centr ugal forces acting in the direction of saidstrands on said first rotor,

h. null-detecting means operatively connected to said transducer meanswhereby when said controlling means adjust the angular velocities ofsaid first and second rotors to cause the centrifugal force acting onsaid first rot r to balance the gravitational force acting on saidrotor, an AC null is indicated, and

i. measuring means for measuring the angular velocity of said firstrotor, the measurement providing an accurate and precise measure ofgravity when said controlling means adjusts the angular velocities ofsaid first and second rotors to produce an AC null in said transducermeans.

2. A gravity meter as described in claim 1 wherein said drive means torotate said first rotor comprises a variable speed motor connected to adrive shaft, said drive shaft being connected to said first rotor andwherein said fixed point lies along the axis of said drive shaft.

3. A gravity meter as described in claim 1 wherein said drive means torotate said second rotor comprises an optical drive system,

a. said system including a plurality of circumferentially spacedprotrusions around the perimeter of said second rotor,

b. each said protrusion having at least one radially directed surfacewhich is blackened so as to absorb radiation with the remaining surfacesbeing nonradially directed and highly polished so as to reflectradiation,

. a random source of radiation directed to impinge on said protrusionswhereby said blackened surfaces absorb radiation and said polishedsurfaces reflect radiation thereby to cause a reactive force of theambient gas molecules near said blackened surfaces to cause said secondrotor to rotate.

4 The gravity meter as described in claim 3 wherein said random sourceof radiation comprises a plurality of randomly directly light sourcesdisposed so as to flood said protrusions with light from all directions.

5. A gravity meter as described in claim 4 wherein said means to controlthe angular velocity of said second rotor comprises means to vary theintensity of radiation emanating from said sources of radiation therebyto vary the reactive forces of said ambient gas molecules tending torotate said second rotor.

6. Apparatus as described in claim 3 including means to accuratelydetermine the angular velocity of said second rotor comprising acoaxially directed beam of light on one side of said second rotordirected to impinge on the periphery of said second rotor and a counteron the other side of said rotor in line with said beam of light, saidcounter being adapted to count and correlate the frequency of theinterruptions to the beam of light caused by the rotation of said secondrotor thereby to indicate the angular velocity of said second rotor.

7. A gravity meter as described in claim 1 wherein said drive means torotate said second rotor comprises,

a. a disk fixed to said drive shaft having a magnetic signature on itssurface,

b. first transducer means in contact with said magnetic signature andoperable to generate a frequency responsive to the angular velocity ofsaid drive shaft,

0. a magnetic signature on said second rotor identical to the magneticsignature on said disk,

d. transducer means on said first rotor in contact with said magneticsignature on said second rotor,

. said transducer means on said first rotor being operably coupled tothe signal output of said first transducer means thereby to impress saidsignal on said second rotor whereby said second rotor rotates withrespect to said first rotor at an angular velocity equal to the angularvelocity of said drive shaft.

4 8. A gravity meter as described in claim 1 wherein said eccentricallydisposed mass on said second rotor comprises a bore within a portion ofsaid second rotor which is filled with a material having a densitygreater than the density of the material on said second rotor.

1. A gravity meter which utilizes centrifugal force of a magnitude thatmay be accurately determined with great precision to balance thegravitational force to be measured comprising: a. a fiRst rotor having afixed and known mass which is free to rotate in a single plane ofrotation about a fixed point, b. a second rotor having a fixed and knownmass including an eccentrically disposed mass, c. said second rotorbeing coaxially disposed about and supported by said first rotor andfree to rotate in a single plane of rotation about said fixed point, d.first drive means for rotating said first rotor, e. second drive meansfor rotating said second rotor and said eccentric weight, f. controlmeans for adjusting the angular velocities of said first and secondrotors, g. transducer means supported by strands located between saidfirst drive means and said first rotor for sensing the resultantcomponent of gravitational and centrifugal forces acting in thedirection of said strands on said first rotor, h. null-detecting meansoperatively connected to said transducer means whereby when saidcontrolling means adjust the angular velocities of said first and secondrotors to cause the centrifugal force acting on said first rotor tobalance the gravitational force acting on said rotor, an AC null isindicated, and i. measuring means for measuring the angular velocity ofsaid first rotor, the measurement providing an accurate and precisemeasure of gravity when said controlling means adjusts the angularvelocities of said first and second rotors to produce an AC null in saidtransducer means.
 2. A gravity meter as described in claim 1 whereinsaid drive means to rotate said first rotor comprises a variable speedmotor connected to a drive shaft, said drive shaft being connected tosaid first rotor and wherein said fixed point lies along the axis ofsaid drive shaft.
 3. A gravity meter as described in claim 1 whereinsaid drive means to rotate said second rotor comprises an optical drivesystem, a. said system including a plurality of circumferentially spacedprotrusions around the perimeter of said second rotor, b. each saidprotrusion having at least one radially directed surface which isblackened so as to absorb radiation with the remaining surfaces beingnonradially directed and highly polished so as to reflect radiation, c.a random source of radiation directed to impinge on said protrusionswhereby said blackened surfaces absorb radiation and said polishedsurfaces reflect radiation thereby to cause a reactive force of theambient gas molecules near said blackened surfaces to cause said secondrotor to rotate.
 4. The gravity meter as described in claim 3 whereinsaid random source of radiation comprises a plurality of randomlydirectly light sources disposed so as to flood said protrusions withlight from all directions.
 5. A gravity meter as described in claim 4wherein said means to control the angular velocity of said second rotorcomprises means to vary the intensity of radiation emanating from saidsources of radiation thereby to vary the reactive forces of said ambientgas molecules tending to rotate said second rotor.
 6. Apparatus asdescribed in claim 3 including means to accurately determine the angularvelocity of said second rotor comprising a coaxially directed beam oflight on one side of said second rotor directed to impinge on theperiphery of said second rotor and a counter on the other side of saidrotor in line with said beam of light, said counter being adapted tocount and correlate the frequency of the interruptions to the beam oflight caused by the rotation of said second rotor thereby to indicatethe angular velocity of said second rotor.
 7. A gravity meter asdescribed in claim 1 wherein said drive means to rotate said secondrotor comprises, a. a disk fixed to said drive shaft having a magneticsignature on its surface, b. first transducer means in contact with saidmagnetic signature and operable to generate a frequency responsive tothe angular velocity of said drive shaft, c. a magnetic signature onsaid second rotor identical to the magnetic signature oN said disk, d.transducer means on said first rotor in contact with said magneticsignature on said second rotor, e. said transducer means on said firstrotor being operably coupled to the signal output of said firsttransducer means thereby to impress said signal on said second rotorwhereby said second rotor rotates with respect to said first rotor at anangular velocity equal to the angular velocity of said drive shaft.
 8. Agravity meter as described in claim 1 wherein said eccentricallydisposed mass on said second rotor comprises a bore within a portion ofsaid second rotor which is filled with a material having a densitygreater than the density of the material on said second rotor.